Below the surface of Manchester, Iowa, lurked a serious problem slowly costing the city significant amounts of money. Not visible above ground, the infiltration and inflow (I/I) of groundwater seeping into the city’s wastewater lines was causing issues with the wastewater treatment plant. If not dealt with immediately, it would continue to cost the city big dollars in the future.
The deteriorating sewer line, located in a flood plain, was allowing significant amounts of I/I to creep in, make its way to the wastewater treatment plant, and ultimately require extra resources to clean the water for reintroduction to the watershed. Along with the I/I issues, the sewer line did not provide easy access for maintenance personnel and was located close to private property and established residential buildings, requiring options other than simple open-trench installation. The City of Manchester turned to city engineer Fehr Graham to help design a solution to replace the line while considering the surrounding areas. Once the project was evaluated, there were additional challenges to meet to provide a successful solution for all involved.
After initially establishing the need for the sewer line to be replaced, several additional challenges were discovered. The wastewater line was installed in 1952 in an undeveloped area of the city, composed of vitrified clay pipe (VCP), and located in a flood plain. As the area developed over the years, residential structures were constructed close to the sewer main. In one location, the sewer main was within 7 feet of a structure and 14 feet below the surface. In other locations, structures were constructed directly over the existing main. The location of the sewer main near residential structures caused an ongoing challenge for maintenance personnel to access.
In addition to the proximity to existing structures, the prevailing water level of a nearby pond was above the installation area. The level of the nearby pond, the fishing season, and the pond’s fish population had to be considered before a resolution could be determined.
The additional I/I had also caused the sewage treatment plant to use additional water and electricity resources to process the excess flow.
Manchester had to find a way to replace the current sewer main to reduce the I/I, provide better maintenance options, and be sensitive to the residents’ homes and property as well as the local environment.
As a result, Fehr Graham and the City of Manchester worked closely with the Iowa Department of Natural Resources (DNR) Fisheries Office to provide a resolution that not only satisfied the residents concerned with the pond’s fish population, but also saved the city the expense of additional dewatering.
A tale of two sections
Fehr Graham engineers worked with the city to develop a solution to upgrading the sanitary sewer trunk main while considering how to reduce I/I, improve access for maintenance, while still being sensitive to surrounding property and the environment. There were two distinct sections of this project, with the solution varying for each:
- The first section, and a majority of the project, included standard open-trench installation of approximately 540 feet of 15-inch main and 1,460 feet of 12-inch main. Most of the new main was located along a different alignment than the original. The new location of the first section created flexibility and resulted in additional space from existing houses and improved access for maintenance personnel.
- The second section, located north of the first section, included a 400-foot main through a residential area. The alignment of the second section couldn’t be altered because of the location. Consequently, open-trench installation would not be possible because of the negative impact it would have on the residential area. Existing houses, garages, and other structures were located too close to the main. A key factor to the solution for the north section was to minimize the impact on the existing residences and structures.
A sanitary sewer trunk main replacement can be a complex undertaking. A high volume of wastewater needed to continue to flow during construction, so the timing of the replacement was a major consideration. The new alignment on the first section of the trunk main allowed construction to proceed while the existing trunk main carried the wastewater flows to be treated. The new alignment provided more distance between private structures and the sewer main, benefiting both the property owner and city as access for maintenance was greatly improved.
Due to the alignment of the second section of sewer main line through the backyards of residential properties, additional complexities arose. Not only was the alignment located in a residential area, but the proximity to residential structures decreased viable options.
The engineers at Fehr Graham creatively addressed the impact and alignment issues of the second section through a process called pipe bursting — a trenchless method for replacing buried pipelines. The project design included bursting 400 linear feet of 14-inch, high-density polyethylene (HDPE) sanitary sewer pipe through the existing 10-inch VCP sanitary sewer line. The process proved successful by limiting the disturbance to nearby residential structures by tunneling through the existing pipe. The pipe bursting process increased the size of the main and, combined with reorienting of the alignment, provided an effective solution to a complex problem.
Another element of complexity was the prevailing water level of an adjacent pond. The water level above the project location would cause inflow and require removal of significant amounts of water, thereby adding expense to the project. The solution was to lower the pond level 18 inches and schedule work during the winter months. This provided just enough relief to eliminate the need for additional dewatering.
Because of the fish population, the Iowa Department of Natural Resources Fisheries Office was also consulted to protect the habitat and satisfy residents. In addition, work was done in a time when local fisherman weren’t affected. This information was brought out in conversations that the city and Fehr Graham had with citizens in the neighborhood.
In addition to the normal challenges associated with sewer main construction, the project had to consider and minimize disturbance to property owners. Communicating to property owners that construction was going to occur in their backyards was not a recipe for buy-in. As a result, property owner outreach was conducted through town hall meetings, personal phone calls, and home visits. The open communication allowed Fehr Graham engineers to fully understand the concerns of property owners, and in turn, provide a design solution that would minimize impact to private properties.
Installation of the main did not begin until the affected property owners had been informed of the project. Although the additional effort by the city and the design staff came at a cost, the return on investment was substantial. The proactive communication process lessened disputes, provided better information on adjacent property areas of concern, and reduced construction questions and delays. This led to quicker project completion and a positive impression on the city.
The sanitary sewer trunk main project was designed and constructed with the future in mind. Using materials with a long service life and increasing the diameter of the pipe will allow the trunk main to last for years, even as the city continues to grow. In addition, the wastewater treatment plant has benefited significantly by the reduced I/I. It now requires less energy to pump and treat wastewater and is producing a high-quality effluent.
Successful completion of the sanitary sewer trunk main replacement project resulted in minimal impact to property owners and the community, in addition to creating goodwill among the residents of Manchester. Finishing the project in advance and under budget, while keeping an open line of communication, have been positive outcomes. In addition, due to the focus on transparency, the partnership with the city enabled innovative thinking in project management, community engagement, and sustainability.
Ryan M. Wicks, P.E., is branch manager, Fehr Graham, in Manchester, Iowa. Contact him at firstname.lastname@example.org.